Inappropriate activation of the Wnt-signaling protein, -catenin causes cancer in a wide range of organs, including ~90% of colorectal cancers. -catenin has also been shown to promote self-renewal of adult stem cells and embryonic stem cells. Several studies suggest that effects of -catenin on stem cells are directly linked to its ability to cause cancer; however the downstream mechanisms connecting Tcf- -catenin effects on stem cells and tumor formation remain a significant question in the field. In order for -catenin to stimulate tumor formation and stem cell self-renewal, it binds to members of the Tcf family of proteins, which function as the DNA-binding effectors of Wnt- -catenin signaling. One Tcf protein, Tcf3, is expressed in several different types of stem cells (hematopoietic, neural, embryonic, hair follicle) that are sensitive to -catenin levels. Our preliminary studies demonstrate that Tcf3 functions as an inhibitor of -catenin in stem cells, suggesting the possibility that Tcf3 has tumor suppressor-like functions. This proposal focuses on the effects of Tcf3 in mice specifically as they relate to stem cell self renewal and tumor formation. We propose two specific aims: (1) to determine the function of Tcf3 in preventing cancer in organs where it is highly expressed (prostate, colon) and in an organ (uterus) that we found to develop tumors in TCF3+/- mice, and (2) to elucidate the downstream molecular mechanisms used by Tcf3 that regulate stem cell self renewal. Examination of the downstream mechanisms (Aim 2) in TCF3 conditional knockout mice (Aim 1) will be used to determine the underlying cause of defects in mice. The proposed experiments will generate important new understanding into the underlying molecular controls of stem cell self renewal and tumor formation. Should they demonstrate that loss of Tcf3 leads to tumor formation, they will identify a potentially novel tumor suppressor gene. Should they reveal that loss of Tcf3 does not confer a susceptibility to cancer, they will identify new avenues for safe cell-based therapeutics to promote stem cell self renewal. PUBLIC HEALTH RELEVANCE: The proposed research will elucidate fundamental mechanisms by which genes in stem cells determine whether stem cells will make more stem cells, make different cell types, or stop dividing. In addition, powerful genetic experiments using mice will determine how defects affecting the molecular control of these fundamental mechanisms in adult stem cells could lead to malignancies in organs such as the uterus, colon and prostate.